Many different types of centrifugal separators are known for separating heterogeneous mixtures into components based on specific gravity. Typically, a heterogeneous mixture, which may also be referred to as feed material or liquid, is injected into a rotating bowl of a centrifugal separator. The rotating bowl spins at high speeds and forces components of the mixture that have a high specific gravity to separate therefrom by sedimentation. As a result, dense solids compress as a cake tightly against an inner surface or wall of the bowl and clarified liquid forms radially inward from the cake. The bowl may spin at speeds sufficient to produce forces 20,000 times greater than gravity so as to separate the solids from the centrate.
As solids accumulate along the wall of the bowl, the clarified liquid exits from the bowl and leaves the separator as “centrate.” Once it is determined that a desired amount of solids has accumulated, the separator is placed in a discharge mode in which the solids are removed from the separator. Often, for example, an internal scraper is engaged to scrape the solids from the walls of the bowl.
Conventional separators have many shortcomings when discharging particular kinds of solids and liquids. For example, some separators may not be capable of completely discharging solids that are sticky, which can result in poor yields. A poor yield can be especially problematic for high-value solids such as those encountered in pharmaceutical processes. Traditional separators also subject a feed material to very high shear forces when accelerating the material to the rotational speed of the bowl, which can damage, for example, sensitive chemical or biological substances such as intact cells.
Still, other separators do not provide a convenient means by which to handle and recover sensitive solids. For example, an operator is commonly used to assist with solids discharge and recovery. Separators that require such operator intervention often suffer from contamination problems. Furthermore, some separators employ numerous mechanical components to facilitate solids recovery, which can affect separator durability. Such components are usually external to the separator or in the form of add-on equipment that poses both size and compatibility issues. Conventional separators also tend to be difficult to clean or sterilize without significantly increasing maintenance costs.
It would be desirable to have a centrifugal separator that can be effectively used with solids of the type described above, namely, those that result in sticky accumulations or are sensitive to shear forces generated during centrifugation. It would also be useful to have a separator that can easily recover such solids without the possibility of external contamination or additional mechanical equipment. Such a separator should also be able to be conveniently cleaned or sterilized-in-place.
Furthermore, typical cross-flow microfiltration systems employ pretreatment of feed liquid to ensure the solids concentration is sufficiently below a threshold at which the filter membrane would become fouled. Backwashing is often necessary to reduce the concentration of accumulated solids on the filter membrane, causing delays in processing. In addition, some mechanism must be provided to extract accumulated solids, such as from a retentate tank. Such mechanisms previously have resulted in removed solids wetted by a relatively significant amount of retentate. The provision of pretreatment liquid and such a solids removal mechanism result in increased processing time, complexity, and cost while providing suboptimal solids drying performance.